Origins of the Invertebrates

When did the first animals evolve?

For roughly 3 billion years after the first prokaryotic cells evolved, all prokaryotes and eukaryotes were single-celled. We don't know when the first multicellular animals evolved from single-celled eukaryotes. Several kinds of data support the hypothesis that animals evolved from ancestors they shared with organisms called choanoflagellates (koh AN uh FLAJ uh layts). These are usually single-celled eukaryotes, but they sometimes grow in colonies. They share several characteristics with sponges, the simplest multicellular animals.

Traces of Early Animals Our oldest evidence of multicellular life comes from recently discovered microscopic fossils that are roughly 600 million years old. The first animals were tiny and soft-bodied, so few fossilized bodies exist. Still, recent studies have uncovered incredibly well preserved fossils of eggs and embryos, such as the embryo in Figure 26–1. Other fossils from this time period have been identified as parts of sponges and animals similar to jellyfish. Paleontologists have also identified what are called “trace fossils” from this time period. Trace fossils are tracks and burrows made by animals whose body parts weren't fossilized. Such fossil evidence indicates that the first animals began evolving long before the Cambrian Explosion.

FIGURE 26–1 Fossil Evidence Fossils such as the 565-million-year-old embryo at left are among the rarest and most valuable treasures that the backbreaking work of hunting for microfossils can yield. (SEM 100X)